In an intake or exhaust vale provided in a cylinder head of an internal combustion engine, the flow of intake air or combustion exhaust gas around the valve members becomes uneven when the intake or exhaust valve is opened. The uneven flow of gas applies a bending stress on the valve members, causing the valve members to contact the valve seats on one side only. Such one-sided contact causes uneven wear on the valve members and the valve seats. The uneven wear may lead to a flow leak between the intake or exhaust valve and the valve seat, in which case the intake or exhaust valve or the valve seat must be replaced.
The exhaust valve, in particular, tends to contact the valve seat on one side only, as it is exposed to high temperature combustion gas flowing therearound so that the valve member and the valve seat are prone to thermal deformation or thermal degradation. Uneven flow or scoring is also caused by carbonized fuel residue adhering on the valve member or the valve seat. Such carbonized substance adhering and depositing between a valve rod and a sleeve that slidably guides the valve rod will hinder smooth sliding of the valve rod.
Moreover, since the combustion gas passing through the exhaust valve has a higher pressure than the intake air through the intake valve, the exhaust valve is subjected to a high bending stress. This bending stress also causes the valve to contact the seat on one side only. Furthermore, when the combustion exhaust gas exits from the cylinder through the exhaust valve into the exhaust port, turbulence is easily generated because the lower surface of the exhaust valve member is not streamlined. Such turbulence also causes uneven flow of gas.
Patent Document 1 and Patent Document 2 disclose means of preventing uneven wear on an intake or exhaust valve and the valve seat in internal combustion engines. The means disclosed in Patent Document 1 is comprised of a plurality of circumferentially spaced grooves on the surface of a valve member facing the intake or exhaust port. Swirls of intake or exhaust gas passing through the intake or exhaust port are blown to these grooves to rotate the intake or exhaust valve by the turning motion of the swirls, to prevent uneven wear on the intake or exhaust valve and the valve seat.
The means disclosed in Patent Document 2 is built upon the design of a cylindrical sleeve that slidably guides an intake or exhaust valve. This means will be described below with reference to FIG. 3 (FIG. 1 of Patent Document 2). FIG. 3 shows an exhaust valve as one example. A cylindrical sleeve 104 is fixed inside the cylinder head 100. The valve rod 102a of the exhaust valve 102 is slidably guided in the sleeve 104. The valve member 102b of the exhaust valve 102 rests on a valve seat 106 by a resilient force of a coil spring (not shown), and moves away from the valve seat 106 by the action of a cam (not shown), thus opening and closing the exhaust port 108.
The sleeve 104 protrudes from the upper surface (reference surface) 100a of the cylinder head 100 by an amount L0, which is more than twice the outer diameter of the valve rod, and the entire length L1 of the sleeve is more than six times the outer diameter S of the valve rod. This restricts inclination of the valve rod 102a so that the exhaust valve 102 does not contact the valve seat 106 on one side only, whereby uneven wear on the exhaust valve 102 and the valve seat 106 is prevented.    Patent Document 1: Japanese Utility Model Application Laid-open No. H6-34101    Patent Document 2: Japanese Utility Model Application Laid-open No. H6-37507
If carbonized substance adheres on the valve rod or the valve seat, or deposits between the sleeve and the valve rod, the turning motion of swirls may not be sufficient to rotate the exhaust valve. Therefore, the means disclosed in Patent Document 1 cannot effectively be applied for exhaust valves.
The means disclosed in Patent Document 2 is not based on the fact that the uneven flow of intake air or combustion exhaust gas around the valve member is the major cause of uneven wear on the valve member or the valve seat. Thus, it does not provide a fundamental solution to prevent uneven wear.